1. Field of the Invention
The present invention relates to a focus detecting (auto-focus) auxiliary or supplementary light projecting device in a passive automatic focusing apparatus which is used with still cameras, movie cameras and other photographic equipment.
2. Background of the Invention
There are two principal approaches to automatic focusing used in present-generation photographic equipment. The methods can be categorized as active or passive. In the active system, a beam of infrared light or ultrasonic waves is projected onto the scene or object to be imaged and the resulting reflected light or echo is received and used to calculate the distance to the object. One problem with this approach is that the range over which distance measurement can be achieved is substantially limited by the maximum distance that can be covered by infrared light or ultrasonic waves. Therefore, automatic focusing of single-lens reflex cameras and other photographic equipment that employs lenses of long focal distance as well as these of short focal distance is chiefly accomplished by the passive method which utilizes available ambient light to directly effect image detection.
Passive automatic focus detection with cameras is most commonly achieved by what is generally referred to as the correlation method. According to this method, a pair of light-receiving devices (e.g. CCDs, i.e., charge coupled devices) each having a plurality of light-receiving areas are used. By comparing the photoelectric output of a light-receiving area in one device with the output from the corresponding light-receiving area in the other device, the point where closest matching between the two outputs occurs is detected and used as the proper focused point. This correlation method, however, is not highly suitable for focus detection in a dark scene because a relatively small amount of light will be emitted from the scene for reception by the light-receiving devices. This approach requires good lighting and contract conditions and sometimes fails if the scene is dark or has a low contrast.
This problem could be solved by flooding the object with auxiliary light from the direction of the camera. However, if auxiliary illuminating light is simply projected from a location in the neighborhood of the optical axis of imaging lenses in the camera, the illumination falls perpendicularly on the object and is reflected from the object to produce a strong specular or skin reflection component, which will return to the camera so as to reduce the contrast of the object.
The auxiliary light should not cause glare to the person in the scene and, to this end, it generally has a relatively long wavelength (e.g. 700 nm) in the low luminosity range where the human eye is practically insensitive. However, the low-contrast phenomenon described in the preceding paragraph becomes pronounced if the object is illuminated with auxiliary light of long wavelength. With a view to avoiding this problem, it has been proposed that a pattern of alternating light and dark vertical lines be projected for providing a contract for the object. Therefore, the auxiliary light projecting devices in current use are designed to deliberately provide contrast for the object by projecting as striped pattern onto the object.
An example of the system that operates on this principle is shown schematically in FIGS. 1 and 2. The system illustrated in these drawings is intended to be used for a camera equipped with a through-the-lens (TTL)-type auto-focusing (AF) mechanism and an imaging lens 10 also serves as a lens in the focus detecting system.
The imaging lens 10 and a projection lens 11 are arranged in such a manner that the optical axis L.sub.1 -L.sub.2 of the imaging lens is parallel to the optical axis l.sub.1 -l.sub.2 of the projection lens 11. A patterned surface 12 that is disposed in a direction perpendicular to the optical axis l.sub.1 -l.sub.2 is offset upwardly by a predetermined amount with respect to the axis l.sub.1 -l.sub.2.
A light source (not shown) is disposed in the back of the patterned surface 12 and light coming from this source passes through the patterned surface 12 and the projection lens 11 to form a patterned image 13 at point C.sub.1 on the optical axis L.sub.1 -L.sub.2 of the imaging lens 10. In FIG. 1, a film surface 14 is located behind the imaging lens 10.
The system shown in FIGS. 1 and 2 produces a focused patterned image 13 if the object is within a very small distance range including point C.sub.1. However, if the object is outside of this range, only a blurred patterned image 13 is produced and the necessary contrast cannot be imparted to the object. With a view to expanding the focusing range of patterned image 13, the F number of the projection lens 11 may be increased so as to provide a greater depth of focus but then, the amount of light available for the patterned image 13 is excessively decreased to put constrains on the range over which proper focus detection can be achieved.
Another problem with the system depicted in FIGS. 1 and 2 is that the gap between the optical axes l.sub.1 -l.sub.2 and L.sub.1 -L.sub.2 causes parallax on account of the distance from the object. As a result, the actual focusing range of this system is limited by whichever is the smaller of the range limited by the pattern dimensions and the range limited by the depth of focus.
Another system that embodies the idea of projecting a striped pattern is shown in FIGS. 3 and 4. In this system, the projection lens 11 and imaging lens 10 are so positioned that the optical axis l.sub.1 -l.sub.2 of the projection lens 11 will cross the optical axis L.sub.1 -L.sub.2 of the imaging at a point C.sub.1 on the imaging optical axis L.sub.1 -L.sub.2. The patterned surface 12 is disposed to cross the projecting optical axis l.sub.1 -l.sub.2 at a right angle so that a patterned image 13 inclined to the imaging optical axis L.sub.1 -L.sub.2 will be formed at point C.sub.1. This system provides a somewhat broader focusing range than the system shown in FIGS. 1 and 2 but it still has the disadvantage that a blurred image of the patterned image 13 will be formed outside of this focusing range.
Therefore, in the actual system available commercially today, two units of the auxiliary light projecting system shown in FIG. 1 are together provided as shown in FIG. 5. In this arrangement, light issuing from a light source (not shown) passes through the patterned surface 12 and projection lens 11 to form a patterned image 13 at point C.sub.1 on the imaging optical axis L.sub.1 -L.sub.2, while light coming from another light source (not shown) passes through another patterned surface 12' and projection lens 11' to form a second patterned image 13' at another point C.sub.2 on the imaging optical axis L.sub.1 -L.sub.2.
This commercial system has the advantage that the focusing range can be expanded to a certain extent while effectively compensating for parallax. However, even this system does not offer a complete solution to the aforementioned problems and the focusing range that can be attained is limited to be within the depths of focus of C.sub.1 and C.sub.2. In addition, a compact system cannot be fabricated because it is necessary to install two projector units in, for example, an accessory strobe apparatus rather than in the camera body itself.
A TTL-type active AF system adapted to a TV zoom lens is shown on page 457 (47) of Kogaku (Optics), 10, 6 published by the Meeting on Optics, the Society of Applied Physics of Japan, December 1981. This system projects auxiliary light through an imaging lens and provides a wide focusing range. However, it requires that an image-focusing lens which matches the imaging lens be provided in front of both the light-emitting and the light-receiving devices. Therefore, the system cannot be readily adapted to a single-reflex lens camera which requires easy lens changing in its operation.